Ideas on tackling moisture in bivys

How were you thinking of adding a window at the foot end? I think you're right, that a window would help a lot for condensation, but how would you do it so as not to let moisture in? At the head end, if it's raining you'll quickly know if any water is coming in, but with a foot window, it might be a while before you realize. But if you could get it to work, you'd not only be ventilating the foot, you'd be creating the possibility of flow-through ventilation.

Adrian,

If you are looking at multi-day trips in very cold weather far below freezing, down towards 5 F, some form of vapor barrier may be the way to go whether you use a bivy or not. At that temperature, no amount of bivy ventilation will prevent moisture from condensing in your sleeping bag when the dewpoint is in the bag itself- this is true for both down and synthetic. A recent thread I participated in looked at vapor barrier liners, and I experimented fairly successfully with bivy and a vapor barrier liner made out of garbage bags.

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=17103&disable_pagination=1

While most find vapor barrier liners less comfortable once the temperatures climb above the low 20s F, some are able to comfortably sleep in them in higher temperatures. There is no doubt that they deal very effectively with internal moisture, but only you can decide whether it fits with your system (VB clothes are also an option). Andrew Skurka's recent article is another great resource.

You've hit on some other great ways to regulate moisture in a bivy- especially keeping it as open as possible. I've seen designs that incorporate an opening in the footbox, but have never used one. In my experience, it is best to take advantage of every opportunity to dry out insulation in the wind or sun, even when it is very cold- this has served me well.

James

Adrian,

Do you have the Momentum or eVent top? With eVent you are probably going to have to do something for extended trips at the temps you are talking about.

With Momentun, maybe or maybe not. Apparently it varies by user and length of trips. I have a Soul Bivy Side Zip with a Monmentum top. I have had no condensation down to 20F. But the longest I have used the bivy is two nights.

I am no fabric expert. As stated on the MLD website, Momentum is water resistant and breathable and eVent is waterproof and "breathable." Which is why I opted for the Momentum.

I have another bivy which is water proof and breathable (yeah, right!). Condensation is an issue with it. But using a Vapor Barrier Liner, there is no condensation.

I would be hesitant about modifying the foot box, and potentially allowing moisture in from the top.

James S – I'm using the MLD Patrol shelter, which has very good protection around the lower end from any rain because the back can sit flush against the ground. Perhaps in a regular tarp that might be more of an issue? But if you're getting the bottom of your bivy steadily rained on, over the course of a night a non-waterproof upper like Momentum isn't going to do the job either.

James D – 5F is probably a minimum, more commonly I imagine the temperature might be perhaps around 20F. I had assumed it wouldn't be cold enough in winters here for VBL (certainly not cold enough during the day).

But perhaps it's something I should look into. Since the foot box is the first to suffer, I wonder if just VBL socks would be worthwhile on colder nights.

Also this raises the issue that adding a window for ventilation could push the dew point inside the bag by lowering the temperature of the bag surface.

Nick – bivy upper is Momentum. As for allowing in moisture from the top, as mentioned, I'm confident the shelter above sufficiently protects the lower end of my bag. And again, I don't think you that can rely on a non waterproof fabric like Momentum for blocking much more than occasional spray anyway.

Adrian,

In theory, in sub-freezing but not frigid temperatures, it is possible that the dewpoint could be inside your bag in some areas of the bag (say, down by the feet) but not others (near your chest). Since your feet give off quite a bit of moisture, and are relatively cool, vb socks might be useful- is worth some experimentation! With a vb liner, you can always cinch it up to your waist and accomplish a similar function but with more coverage.

VB deals with internal moisture, and bivy fabric will have very little impact on this issue in very cold temps. When using a vb liner, the only thing you are asking the bivy to do is keep moisture out- its breathability does not really matter. Of course, there are other conditions where bivy fabric is very important, and I, like you, have the mld soul bivy because, for most of my trips, I want a very breathable bivy.

Another thing to keep in mind is that, when using a bivy w/o vb, sometimes the inner surface of the bivy becomes the condensation point- I can recall nights when I had frost on the inner bivy surface but not on the sleeping bag (at least not any visible condensation). Get rid of that frost before it thaws, and it can be a big help.

James

In reference to adding a vent at the foot, I've thought of doing the same thing. The only way I can think of doing it without letting rain in is to make a little fly that covers an insect window. Getting much air circulation down there would mean adding a hoop or a guyline for volume, though, and I don't like to spend much time pitching my bivy.

I've also thought it would make sense to build a bivy/sleeping bag hybrid to deal with such issues: construct the part that covers the lower 2/3 of the body like a water-resistant sleeping bag, and design the rest so it functions like a bag with a bivy cover. I'm not sure if the design would work, but it would eliminate a layer that collects moisture.